The process of cutting continuous reinforcement fibers into discrete length reinforcement fibers is useful in the manufacture of different types of reinforcement structures. For example, the discrete length reinforcement fibers can be used in reinforcement mats such as mats made with commingled fibers (e.g., carbon fibers commingled with thermoplastic fibers), or laminated mats made from layers of fibers.
The discrete length reinforcement fibers can also be used in reinforcement preforms. Structural composites and other reinforced molded articles are commonly made by resin transfer molding and structural resin injection molding. These molding processes have been made more efficient by preforming the reinforcement fibers into a reinforcement preform which is the approximate shape and size of the molded article, and then inserting the reinforcement preform into the mold. To be acceptable for production at an industrial level, a fast preforming process is required. In the manufacture of preforms, a common practice is to supply a continuous length of reinforcement strand or fiber to a reinforcement dispenser (or "chopper"), which cuts the continuous fiber into many discrete length fibers, and deposits the discrete length fibers onto a collection surface. This process can be used to make preforms in an automated manner by mounting the reinforcement dispenser for movement over the collection surface, and programming the movement of the dispenser to apply the reinforcement fibers in a predetermined, desired pattern. The reinforcement dispenser can be robotized or automated, and such reinforcement dispensers are known art for such uses as making preforms for large structural parts, as in the auto industry, for example. (Dispensers of reinforcement fibers for the manufacture of mats of commingled fibers or laminated mats can also be adapted to be moveable and programmable.) Typically, the deposited fibers are dusted with a powdered binder, and compressed with a second perforated mold. Hot air and pressure sets the binder, producing a preform of reinforcement fibers which can be stored and shipped to the ultimate molding customer which applies resin to the preform and molds the resinated preform to make a reinforced product, typically using a resin injection process.
As the technical requirements for reinforcement structures increase, new methods for dispensing and laying down reinforcement fibers are required. One requirement is that the reinforcement fibers be delivered at faster speeds than used previously. Another requirement is that the reinforcement fibers be laid down in a predetermined orientation. The advancement in the reinforcement technology enabling a moveable and programmable reinforcement dispenser has led to requirements for very sophisticated fiber patterns and orientations. Reinforcement structures can be designed with specific amounts and of reinforcement fibers to improve the strength of the structure precisely at the weakest or most stressed location of the article to be reinforced. Because of this new sophistication, there often is a requirement that the fibers be laid onto the collecting surface in a closely spaced, parallel arrangement.
Previous efforts to deliver closely spaced, parallel fibers have not been successful, especially at the high speeds necessary for commercial operations. When typical reinforcement dispensers are operated at a faster speed, the resulting discrete length reinforcement fibers cannot be successfully laid down in a parallel, closely spaced orientation. The fibers are directed toward the collecting surface in a direction generally perpendicular to the collection surface, and this procedure does not tend to leave the fibers parallel and closely spaced. Further, typical nozzle-type reinforcement dispensers use an air flow to guide the reinforcement fiber into engagement with the cutting blade, and to dispense the is discrete length fibers after cutting, thereby introducing turbulence to the collection surface which disturbs the orientation of the fibers.
Previous patents also describe methods for dispensing reinforcement fibers which are not successful in dispensing the fibers in a parallel orientation at high speeds. For example, both U.S. Pat. No. 4,169,397 to Vehling and Russian Pat. No. 1,694,724 to Zhitomirskii disclose winding a continuous length of a reinforcement fiber around a circular form to make circular coils, and then cutting the circular coils into discrete length reinforcement fibers. The resulting fibers are dispensed in a random orientation instead of a parallel orientation.
In contrast to the previous efforts, co-pending U.S. application Ser. No. 08/419,621, filed Apr. 10, 1995, discloses a method for dispensing reinforcement fibers which successfully dispenses the fibers in a parallel orientation at high speeds. In the disclosed method, a continuous length of a reinforcement fiber is wound into elongated coils around a form, and then the elongated coils are cut into discrete length reinforcement fibers. The resulting fibers are dispensed in a parallel orientation.
However, there is still a need for an improved method for dispensing reinforcement fibers in a parallel orientation which allows the fibers to be dispensed even more rapidly, so that production on an industrial level can be even more efficient. There is also a need for an improved method for dispensing reinforcement fibers which is gentler on the fibers, so that different types of fibers can be used which are too brittle or too weak to dispense without breaking by previous methods.